Hameg HM1008-2 Manual

Hameg HM1008-2 Manual Manual pdf 21 page image

21Subject to change without noticeCombiScope®The oscilloscope HM1008-2 combines two oscilloscopes inone: An analog oscilloscope and a digital oscilloscope. With atouch of the Analog/Digital pushbutton you can switch betweenanalog and digital mode (oscilloscope operation). To avoid longexplanations, the terms analog and digital mode are used inthe following text.HAMEG oscilloscopes are either analog or they are Combi-Scopes, i.e. they contain a complete analog scope and theadditional hardware and software to sample, digitize, store,process and display the signals. The HM1008-2 is a 150 MHz1 GSa/s CombiScope®.With a HAMEG CombiScope® the user is always sure: he needsonly to switch from digital to analog in order to see the truesignal. This is especially important when a signal is to be docu-mented in digital mode. The user of a pure digital oscilloscopeneeds to know the signal better than the scope!The advantages of digital operation are:– Capture and storage of single events– No flicker with very low frequency signals– Fast signals with a low rep rate or low duty cycle can bedisplayed at high intensity– Due to the storage of all signals they may be easily docu-mented and processed.– High quality crt’s and custom electronic parts are used.The disadvantages of digital operation are:– An analog scope displays the signal itself in real time. Ina digital oscilloscope the signal is not displayed but only alow frequency reconstruction of the signal. The limitationsand problems of sampling operation as well as those ofanalog/digital conversion hold. The display can not be inreal time as, after capturing a signal, the digital oscilloscopemust take a short time to perform calculations the result ofwhich will then be displayed later.– Therefore the capture rate of ordinary digital oscilloscopesis orders of magnitude lower than that of any analog scope.Hence a digital oscilloscope is least suited to catch rareevents.– There is no information in the trace, the trace is always ofequal intensity. Thus valuable information (so called Z axis)is lost. Also the fast slopes of a pulse which are invisible onan analog scope will be of the same intensity as the slowerparts of the signal, this is a gross misrepresentation. Thereason is that digital oscilloscopes ordinarily do not showonly the sampled points but they interpolate by drawing acontinuous trace.– The vertical resolution is mostly only 8 bits. In an analogscope there is no loss of fine detail by digitizing. Even if thetrace is not very crisp details can be seen in it.– Due to the sampling and the lack of a low pass filter in theinput frequencies above half the sampling frequency willcause so called aliases, i.e. low frequency ghost signals.Sampling is practically the same as frequency conversionor multiplication, it creates sum and difference frequencies,beat frequencies which may be orders of magnitude lowerthan the signal frequency and gives grossly erroneous re-sults. In practice, therefore, only frequencies 1/10 or lessof the sampling frequency can be reliably displayed. Themeaning of the Nyquist theorem is mostly misunderstood:if the sampling frequency is only twice the signal frequencythere will only be two points displayed on the screen: anynumber of signal shapes may be drawn which fit throughthese two points. The Nyquist theorem assumes that thesignal is a sine wave. It is easily understood that, in orderto depict an unknown signal shape one needs at least 1 or2 points per centimeter; in other words: the useful signalfrequency is only 1/10 to 1/20 at best.– An analog scope has a frequency response which followsclosely the Gaussian curve, this means in practice that alsofrequencies far beyond the –3 dB frequency will be shown,reduced in amplitude, but they will be shown. This not onlypreserves fine detail of a signal but it allows also to see, e.g.,very high frequency wild oscillations in a circuit. This is notthe case after sampling because all frequencies beyond halfthe sampling frequency will be „folded“ back into the lowerfrequency band.– Due to limited memory depth the maximum sampling ratemust be reduced in a digital oscilloscope when the timebase is set to slow sweep speeds, it may be reduced fromGSa/s to kSa/s! Most users are not aware of this drawback,they think that if they bought a digital oscilloscope with 100MHz bandwidth and 1 GSa/s they are safe when measuringkHz range signals. But such low frequency signals may bedistorted and possibly aliases displayed.Please note: This list of disadvantages is by far incomplete!It only scratches the surface.There are 3 methods of sampling:1st Real time samplingHere the Nyquist theorem must be observed, but, as mentio-ned, in practice the signal frequency is far less than 1/10 thesampling frequency. Consequently, with a 1 GSa/s rate signalswith up to 100 MHz can be adequately reconstructed. Obviously,this is the only mode for single event capturing.2 nd Equivalent time samplingThis is the normal operating mode for all sampling scopes.(Sampling scopes are very old, they are still the fastest scopeswith bandwidths > 50 GHz because they have no input amplifier.Sampling scopes are far superior to digital oscilloscopes be-cause their Y resolution is identical to that of an analog scope).In this mode consecutive periods of the signal are sampled,each period contributes but one sample. The signal period isthus scanned and very many periods are necessary in order toachieve one full screen display. This way a very high „effective“sampling rate is achieved, this method exchanges bandwidthfor time. In a sampling scope a very accurate display is createdwhich is, as far as the shape is concerned, almost as good asthat of an analog scope. In a digital oscilloscope, however, thesample points are 8 bit a/d converted, losing resolution. Thebandwidth achieved is given alone by the hf properties of theinput and the minimum realizable duration of the sampling pul-se, so 14 GHz at a sensitivity of 2 mV/cm and 50 Ω was standardin the 1960’s. In a digital oscilloscope, however, which shouldbe used like an analog scope, a high impedance (1 MΩ) widerange (e.g. 1 mV/cm to 20 V/cm) attenuator must be includedand also an input amplifier. This is why a digital oscilloscopecannot reach the bandwidths of sampling scopes. Equivalenttime sampling suffers fully from the problems of aliasing. As itrequires the (not necessarily periodic) repetition of the signal inC o m b i S c o p e